Radio communication devices transmit radio frequency (RF) communication signals using an antenna. The transmitter of a radio communication device includes a power amplifier to amplify the communication signals before they are coupled to the antenna. For portable radio communication devices that are powered by a battery, operating the power amplifier at high efficiency is important to allow the communication device to operate for long periods of time. However, when most RF power amplifiers are operated in their most efficient manner, they provide non-linear amplification. This means that a change in the amplitude of the signal sent into the power amplifier results in a non-proportional change in the amplitude of the signal out of the amplifier. For constant envelope radio frequency communication techniques such as frequency modulation (FM) this is not a problem but for other modulation techniques such as quadrature amplitude modulation (QAM) non-linearity in the output of the power amplifier output is not acceptable.
One method for linearizing the output of a power amplifier is to use a Cartesian feedback loop such as the one shown in FIG. 1. The use of a Cartesian feedback loop for linearization is described in “Transmitter Linearization Using Cartesian Feedback For Linear TDMA Modulation” by M. Johansson and T. Mattson as published in the proceedings of the 41st Vehicular Technology Conference, May 1991, pages 439-444. The loop of FIG. 1 contains a summer 103, a loop filter 105, a first mixer 107, a power amplifier 113, a radio frequency coupler 115 and a second mixer 119. The portion of the loop containing the loop filter 105, mixer 107 and power amplifier 113 is referred to as the forward path of the loop and the portion of the loop containing the radio frequency coupler 115 and second mixer 119 is referred to as the feedback path of the loop. The signal from the feedback path is subtracted from the communication signal to be transmitted, xi(t), in the summer 103. The signal out of the summer 103 passes through the loop filter 105 and into the first mixer 107 where it is modulated up to radio frequency by multiplication by the output of a oscillator 109. The first mixer 107 output is then amplified by the power amplifier 113 and the resulting signal is sent to an antenna 117. The radio frequency coupler 115 retrieves a portion of the signal coming out of the power amplifier 113 and passes it to the second mixer 119. The second mixer demodulates the signal back down to baseband by multiplying it with the output of the oscillator 109 after it has been phase adjusted.
Cartesian feedback loops can be characterized by a number of different types of frequency responses. The forward frequency response, a(jω), is the frequency response of the forward path of the feedback loop and the feedback frequency response, b(jω), is the frequency response of the feedback path of the feedback loop. The loop frequency response, a(jω)*b(jω), is the product of the forward and feedback frequency responses. FIG. 8 shows an example of a loop frequency response of a Cartesian feedback loop. The loop frequency response, a(jω)*b(jω), is made up of a gain response 805 and a phase response 809. The gain response 805 of the loop frequency response is also called the loop gain. Vertical axis 815 corresponds to the gain response 805 and is in decibels while vertical axis 820 corresponds to the phase response 809 and is in degrees. Horizontal axis 822 represents the logarithm of frequency. At a particular frequency, the amount of distortion that a Cartesian feedback loop can correct is less than or equal to the magnitude of loop gain.
A Cartesian feedback loop can also characterized by its loop bandwidth, phase margin and gain margin. Loop bandwidth 825 is defined as the frequency at which the gain response 805 of the loop frequency response equals 0 dB. Generally, in a feedback loop at frequencies less than the loop bandwidth, the magnitude of the forward frequency response |a(jω)| is much greater than the magnitude of the feedback frequency response |b(jω)|. Phase margin 830 is defined as 180 degrees minus the absolute value of the phase response 809 of the loop frequency response at the frequency where the loop gain is 0 dB. Gain margin 835 is defined as the negative of the gain response 805 of the loop frequency response at the frequency where the phase response 809 is −180 degrees.
One important consideration of Cartesian feedback loop design is stability. Generally, there are two criteria for stability of a Cartesian feedback loop. First, the gain margin must be greater than 0 dB. Secondly, the phase margin must be positive. A more detailed discussion of stability of Cartesian feedback loops can be found in “The Design of CMOS Radio Frequency Integrated Circuits” by Thomas Lee, Cambridge University Press, 1998. Another important consideration of Cartesian feedback loop design is noise performance. Generally, noise performance of Cartesian feedback loops can be improved by keeping the loop bandwidth small. Of course, the loop bandwidth must still be made large enough to pass the communication signal being transmitted. A more detailed discussion of noise considerations in Cartesian feedback loops can be found in “Noise Performance of a Cartesian Loop Transmitter” by Peter B. Kennington, Ross J. Wilkinson and Kieran J. Parsons as published in the IEEE Transactions on Vehicular Technology, Vol. 46, No. 2, May 1997.
The loop bandwidth, phase margin, gain margin and maximum loop gain are functions of the loop filter and gain of the amplifiers in the Cartesian feedback loop. The components of the feedback loop are chosen to make the loop bandwidth large enough to pass the communication signal but small enough to attenuate noise while maintaining stability and providing a large maximum loop gain.
Oftentimes, the components of the Cartesian feedback loop except for the power amplifier and large capacitors associated with the loop filter are implemented in an integrated circuit. Generally, the implementation of the feedback loop in an integrated circuit allows the size and cost of the radio communication device to be reduced relative to circuit designs not employing an integrated circuit. Nevertheless, while it is relatively inexpensive to produce an integrated circuit once it has been designed, the design of an integrated circuit containing a Cartesian feedback loop is a time consuming and expensive process. Also, the cost of producing an integrated circuit is in general inversely proportional to the volume of the integrated circuit produced. Hence it is desirable to use a particular integrated circuit in as many radios as possible to reduce the cost of the integrated circuit by increasing the number produced.
There are many different types of radio communication devices in use today. These types include for example, global system for mobile communication (GSM) radios, code division multiple access (CDMA) radios, IS136 radios, integrated dispatch enhanced network (IDEN) radios and terrestrial trunked radio (TETRA). Generally, each of these different types of radios requires a different loop bandwidth and hence a different design for the Cartesian feedback loop. Dual mode radio communication devices that can function as multiple types of radio communication devices are becoming more common. For example, one radio communication device may function as both a GSM and an IDEN radio communication device. It would be desirable to implement Cartesian feedback loops in such radio communication devices without the need for additional parts.